Impedance tuning circuit

ABSTRACT

A feedback system such as a phase locked loop (PLL) includes a second feedback loop which responds when a VCO control voltage is near either end of its range, by slowly adjusting additional tuning elements which control the VCO frequency. The second feedback loop is arranged to cause a slow enough change in the VCO frequency that the first traditional feedback loop adjusts the control voltage quickly enough in a direction toward its mid-range value to keep the VCO frequency substantially unchanged. The second feedback loop advantageously incorporates one or more digital control signals which preferably change no more than one bit at a time and with a controlled slow ramp rate. As a result, the PLL maintains phase accuracy so that the operation of the PLL, including subtle specifications such as input data jitter tolerance or output jitter generation when used for clock and data recovery applications, is not negatively impacted. An impedance tuning feedback system provides a resistance between two nodes which is proportional to a reference resistance, and preferably incorporates slow digital switching to result in near perturbation-free state changes over the tuning range of the resistance.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is entitled to the benefit of U.S. Provisional Application No. 60/300,699 by Baird, et al. filed Jun. 25, 2001, the disclosure of which is incorporated herein by reference.

This application is related to U.S. patent application No. 10/180,833 by Baird, et al., entitled “Feedback System Incorporating Slow Digital Switching for Glitch-Free State Changes”, filed on Jun. 25, 2002.

BACKGROUND OF THE INVENTION

The present invention relates to feedback control systems, and particularly to those feedback systems utilizing phase-locked loops.

Feedback systems are well known in the art and are found in a multitude of different configurations. One such well-known configuration is a phase locked loop (PLL). A generalized block diagram is shown in FIG. 1 of a traditional PLL configured for a clock and data recovery application. Such a configuration may be used for recovering clock and data streams compatible with the SONET specification, as well as others. The phase locked loop 100 includes a phase detector 102 (or alternatively, a phase/frequency detector) which receives the input data signal conveyed on node 112 and receives a data clock signal conveyed on node 122. The phase detector 102 generates on its output node 116 an error signal which is a function of the phase difference (and frequency difference in the case of a phase/frequency detector) between the input data signal and the data clock signal, and often includes data retiming circuitry to generate on an output node 114 the reconstructed data, as shown.

A loop filter 104 filters the output of the phase detector 102 to generate a control voltage signal on node 118 which is provided to a voltage controlled oscillator 110 in order to influence the frequency (and hence the phase) of the VCO output clock signal conveyed on node 120. The loop filter 104 frequently includes an integrator block which is implemented using a charge pump and a loop filter capacitor. The VCO output clock signal may be divided-down by divider 106 to generate the data clock signal (conveyed on node 122) based upon the expected data rate of the incoming data signal.

If such a PLL were implemented using discrete components, precision components could be used to provide a nominal VCO frequency relatively close to a desired center frequency. However, such a discrete implementation is costly and requires a large amount of printed wiring board space, and more than likely would have difficulty achieving the performance required of modem systems while operating at an acceptable power level. Consequently, most VCOs are implemented monolithicly (i.e., on a single integrated circuit die). As is well known in the art, the absolute value of certain parameters on an integrated circuit may vary greatly due to process variations (e.g., lot-to-lot variations, wafer-to-wafer variations within a lot, die-to-die variations within a wafer) and as environmental variables change (e.g., die temperature, power supply voltage variations, etc.). Even though the tracking of certain parameters within a single integrated circuit is frequently quite good (which is the basis of many advantageous circuit techniques), the nominal frequency of many VCO circuits can vary greatly from die to die. While the frequency of the VCO can inherently be adjusted by an appropriate control voltage, the subsequent adjustability of the VCO may be reduced if the control voltage otherwise necessary to achieve the initially-desired VCO frequency falls too close to either the upper extreme or the lower extreme of its range. Said differently, such a PLL 100 may perform more optimally over time when the control voltage for the VCO is nominally somewhat centered within its expected voltage range.

One possible technique increases the gain of the VCO so that large changes in VCO frequency may be achieved by changes in the control voltage well within the expected range of control voltages. In principle this would allow a PLL to compensate for a large deviation in VCO “center frequency” without requiring a control voltage dangerously close to “running out of range.” But there are detrimental consequences of increasing the VCO gain, including danger of locking onto a harmonic, and increased noise and jitter of the system. Moreover, with most VCO circuit structures it is difficult to arbitrarily provide an ever higher and higher tuning range and still achieve good frequency and phase stability.

One approach to accommodating the VCO center frequency variations involves trimming the frequency using, for example, a precision laser. After the semiconductor fabrication steps are complete, and either during wafer-level testing or possibly after singulization of individual circuit dies, the VCO is tested to determine its center frequency, and various circuit elements (e.g., resistors, capacitors) are trimmed to adjust the center frequency to the desired value. The remaining testing and packaging operations are then performed to complete the manufacturing of the circuits. Alternatively, such trimming may also be accomplished using a flash memory programming technique coupled with appropriate selection circuits, although this requires a semiconductor process capable of forming compatible flash memory elements. In either case, such trimming is a “permanent” adjustment of the center frequency during manufacture, but it adds costly manufacturing steps to either accomplish laser trimming after wafer fabrication or to provide a semiconductor process capable of implementing flash memory structures or other kinds of programmable structures. Moreover, such trimming is performed once during manufacture, and cannot adjust for subsequent changes in environmental conditions that the circuit may be called upon to operate under.

Another approach to accommodating the VCO center frequency variations involves calibrating the VCO center frequency each time the circuit is powered-up. Such techniques may involve comparing the center frequency against an externally provided reference frequency signal and setting a number of storage elements (e.g., registers) to appropriately adjust the center frequency. Such storage elements are volatile and lose stored data when the circuit loses power. An example of a device that performs such a calibration upon power-up is the Si4133G RF Synthesizer, which is available from Silicon Laboratories, Inc. based in Austin, Tex.

These approaches are valuable additions to the state of the art, but they cannot accommodate variations in the center frequency as environmental conditions change, as semiconductor parameters drift over time (e.g., threshold voltage shifts), or as other artifacts of component aging occur. This becomes increasingly more important in certain industrial systems which are put into operation and virtually never shut down. Examples include various interface circuits within the telecommunications infrastructure, which may operate for years without an opportunity to recalibrate during a subsequent power-up operation.

What is needed is an effective way to accommodate environmental or parametric changes in a feedback system which occur after the system is powered up and while operational, without negatively impacting the operation of the feedback system within its intended specifications.

SUMMARY OF THE INVENTION

Such a capability may be accomplished in a clock and data recovery circuit, such as PLL 100, by introducing a second feedback loop which reacts to the control voltage reaching a level sufficiently different from its mid-range value by slowly adjusting additional tuning elements which, along with the tuning elements controlled by the control voltage signal, control the VCO frequency. Absent the first traditional feedback loop, a change in the VCO frequency would result, but the second feedback loop is arranged to cause a slow enough change in the VCO frequency that the first feedback loop is able to compensate for the would-be change by adjusting the control voltage quickly enough in a direction toward the mid-range value to keep the VCO frequency substantially unchanged. The second feedback loop may advantageously incorporate one or more digital control signals to affect a tuning change by changing from one state to another state. These digital signals preferably change no more than one bit at a time, and the single bit which changes is preferably caused to achieve a controlled transition time (or ramp rate) which is slow enough to cause a very gradual change in the value of the associated tuning elements. As a result, the PLL maintains lock during and after the digital signal changes state, and more importantly, maintains its phase accuracy so that the operation of the PLL, including subtle specifications such as input data jitter tolerance and jitter generation are not negatively impacted.

In a broader context of the present invention, such a capability may be accomplished in an electronic feedback system having a first feedback loop responsive to a control signal for controlling an operational parameter of the feedback system, by providing a second feedback loop for generating at least one digital control signal to influence the feedback system in a manner which, absent the first feedback loop, would result in a change to the controlled parameter when the digital control signal changes value. The digital control signals are arranged to influence the feedback system slowly enough, relative to the first feedback loop, to allow the first feedback loop to adjust the control signal and maintain the controlled parameter substantially unchanged as the digital control signal changes value.

In one aspect the invention provides a controlled resistance circuit forming a resistance between a first node and a second node having a controlled magnitude proportional to a reference resistance. The circuit includes a first variable resistance circuit connected between the first node and the second node, having an analog portion responsive to an analog control signal and having a digital portion responsive to a digital control signal, and whose magnitude is responsive to both the analog control signal and the digital control signal. The circuit also includes a second variable resistance circuit connected between a third node and a fourth node, having an analog portion responsive to the analog control signal and having a digital portion responsive to the digital control signal, and whose magnitude, for a given value of the analog control signal and a given value of the digital control signal, is proportional to the magnitude of the first variable resistance circuit. A first feedback circuit is responsive to a voltage developed across the second variable resistance circuit by a controlled first current flowing therethrough, for generating the analog control signal accordingly to adjust the magnitude of the second variable resistance circuit toward a magnitude which is proportional to the reference resistance. A second feedback circuit is responsive to the analog control signal when the analog control signal is driven outside a predetermined range, for generating the digital control signal to accordingly further adjust the magnitude of the second variable resistance circuit in like direction as that resultant from the analog control signal being driven outside the predetermined range, so that the first feedback circuit drives the analog control signal back within the predetermined range. The first and second feedback circuits are preferably arranged to generally maintain the analog control signal within the predetermined range and to adjust the magnitude of the second variable resistance circuit to a magnitude proportional to the reference resistance, thereby likewise adjusting the magnitude of the first variable resistance circuit to a controlled magnitude which is proportional to the reference resistance.

The reference resistance may be a resistor external to an integrated circuit upon which the controlled resistance circuit is implemented, but may also be integral to an integrated circuit upon which the controlled resistance circuit is implemented, and may further be trimmable during manufacture or testing of the integrated circuit.

In yet another aspect the invention provides a resistance circuit providing a resistance between a first node and a second node having a controlled magnitude proportional to a reference resistance. The resistance circuit includes a first variable resistance circuit connected between the first node and the second node, having a magnitude that is responsive to at least one control signal. The resistance circuit includes a second variable resistance circuit connected between a third node and a fourth node, having a magnitude that is responsive to the at least one control signal and which magnitude, for a given value of the at least one control signal, is proportional to the magnitude of the first variable resistance circuit. A bias circuit includes a first bias resistance, and generates a controlled reference current and a controlled bias current, wherein a ratio of the controlled bias current to the controlled reference current is proportional to the ratio of the reference resistance to the first bias resistance. The resistance circuit includes a second bias resistance having a magnitude which is proportional to that of the first bias resistance and which is implemented to track variations in the first bias resistance. A feedback circuit is arranged to compare a first voltage developed across the second bias resistance by the controlled bias current flowing therethrough against a second voltage developed across the second variable resistance circuit by the controlled reference current flowing therethrough, and to generate accordingly the at least one control signal to adjust the value of the second variable resistance circuit so that the first voltage substantially equals the second voltage, thereby resulting in the second variable resistance circuit having a magnitude proportional to the reference resistance. The first variable resistance circuit, being also responsive to the at least one control signal, likewise achieves a magnitude proportional to the reference resistance.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention may be better understood, and its numerous objects, features, and advantages made apparent to those skilled in the art by referencing the accompanying drawings.

FIG. 1, labeled prior art, is a block diagram of a traditional phase locked loop feedback system arranged to recover clock and data from an incoming serial data signal.

FIG. 2 is a block diagram of a clock and data recovery phase locked loop feedback system in accordance with the invention.

FIGS. 3A and 3B are schematic diagrams of an LC tank circuit useful for the present invention.

FIGS. 4A, 4B, and 4C are schematic diagrams of a variable capacitance circuit and its modeling.

FIGS. 4D and 4E are graphs showing characteristics of the modeled circuit element shown in FIG. 4C.

FIG. 5 is a schematic diagram of an LC tank circuit which shows a variable capacitance circuit useful for the present invention which receives a plurality of expanded voltage signals.

FIG. 6 is a waveform diagram of exemplary expanded voltage signals useful for the circuit shown in FIG. 5.

FIG. 7 is a schematic diagram of a tank circuit including a variable capacitance circuit useful for DC calibration of the oscillation frequency of the tank circuit.

FIG. 8 is a schematic diagram of a tank circuit including a variable capacitance circuit useful for gradual adjustment of the oscillation frequency of the tank circuit, in accordance with an embodiment of the present invention.

FIG. 9 is a schematic diagram of a gate control circuit useful for the circuit shown in FIG. 8.

FIG. 10 is a schematic diagram of another gate control circuit useful for the circuit shown in FIG. 8.

FIG. 11 is a schematic diagram of a circuit in accordance with an embodiment of the present invention.

FIG. 12 is a schematic diagram of a circuit useful for the circuit shown in FIG. 11.

FIG. 13 is a waveform diagram of various signals of the circuit of FIG. 12.

FIG. 14 is a schematic diagram of a portion of a VCO circuit useful for the circuit shown in FIG. 11.

FIG. 15 is a schematic diagram of a circuit useful for the circuit shown in FIG. 11.

FIG. 16 is a schematic diagram of an embodiment of a voltage expansion circuit.

FIG. 17 is a schematic diagram of a circuit useful for the circuit shown in FIG. 11.

FIG. 18 is a schematic diagram of a circuit embodiment in accordance with the present invention.

FIG. 19 is a block diagram of an exemplary clock and data recovery phase locked loop feedback system.

FIG. 20 is a schematic diagram of an exemplary variable resistance circuit.

FIG. 21 is a block diagram depicting several variations of a feedback system, all in accordance with the present invention.

FIG. 22 is a block diagram of a feedback system in accordance with the present invention.

FIG. 23 is a block diagram of another feedback system in accordance with the present invention.

FIG. 24 is a block diagram of yet another feedback system in accordance with the present invention.

The use of the same reference symbols in different drawings indicates similar or identical items.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Referring now to FIG. 2, a generalized block diagram is shown of an exemplary PLL 130 configured for a clock and data recovery application which is similar in many respects to the earlier described PLL 100, but which incorporates a second feedback loop to better accommodate gradual environmental or parametric changes in the PLL. The phase locked loop 130 includes a phase detector 102 which generates on its output node 116 an error signal which is a function of the phase difference between the input data signal and the data clock signal, and which includes data retiming circuitry to generate on an output node 114 the reconstructed data, as shown. Unless the context so requires, as used herein a “phase detector” may also refer to a phase/frequency detector. A loop filter 104 filters the output of the phase detector 102 to generate a control voltage signal on node 118. A VCO 136 is provided which includes two separate control inputs (nodes 118 and 134), and which generates on its output node 120 a VCO clock signal. If the PLL 130 is configurable for operation at more than one data rate, the VCO output clock signal is preferably divided-down by divider 106 (based upon the expected data rate of the incoming data signal as communicated by a data rate signal conveyed on node 137) to generate the data clock signal conveyed on node 122. Advantageous circuits for the phase detector 102 and for the loop filter 104 are described in U.S. Provisional Application No. 60/217,208 entitled “Digitally-Synthesized Loop Filter Capacitor,” filed Jul. 10, 2000, which application is incorporated herein by reference in its entirety, and in U.S. patent application Ser. No. 09/902,542 entitled “Digital Phase Detector Circuit and Method Therefor,” filed Jul. 10, 2001, which application is incorporated herein by reference in its entirety, and in U.S. patent application Ser. No. 09/902,541 entitled “Digitally-Synthesized Loop Filter Circuit Particularly Useful for a Phase Locked Loop,” filed Jul. 10, 2001, which application is incorporated herein by reference in its entirety.

The control voltage signal conveyed on node 118 is coupled to the first control input of the voltage controlled oscillator 136 in order to influence the frequency (and hence the phase) of the VCO output clock signal. The control voltage signal is also conveyed to a digital control block 132 which generates a control signal on node 134 which is coupled to the second control input of the voltage controlled oscillator 136. Both control inputs influence the frequency (and hence the phase) of the VCO output clock signal, as is described in greater detail herebelow.

To better describe the operation of various exemplary embodiments of the present invention, a brief digression is warranted. Referring now to FIG. 3A, a resonant LC tank circuit 140 is shown having a variable capacitor 144 connected in parallel with a fixed inductor 142 and together coupled between nodes 146 and 148. Such a resonant circuit is particularly useful in many VCO circuits which use differential circuitry, and may be connected between a pair of differential nodes of the VCO such as, for example, differential nodes of a gain block. As a balanced differential circuit, a virtual ground may be visualized “in the middle” of the variable capacitor 144. As shown in FIG. 3B, the single variable capacitor 144 may be replaced with a pair of variable capacitors 144 a and 144 b respectively connecting nodes 146 and 148 to a common ground node 150.

Compared to a variable capacitor literally connecting two circuit nodes together, a variable capacitor connecting a circuit node to ground is much easier to implement. One implementation of variable capacitor 144 a is shown in FIG. 4A, which includes a fixed capacitor 160 connected in series with an N-channel switch transistor 162 whose source terminal is connected to ground. A variable gate voltage V_(GATE) is applied to the gate terminal of transistor 162 to control the conductivity of the transistor 162. The range of gate voltages as well as the particular transistor threshold voltage may both be chosen to provide a switch transistor that is fully turned off at one extreme of the gate voltage range, and substantially turned on (i.e., conductive) at the other extreme of the gate voltage range. The transistor 162, in effect, functions as a variable resistor (as well as a switch). Consequently, the implementation of variable capacitor 144 a may be modeled, as depicted in FIG. 4B, as a variable resistance 164 in series with the fixed capacitor 160. For a narrow frequency band around the VCO oscillation frequency, this series circuit in turn may be modeled by an equivalent parallel circuit, as shown in FIG. 4C. This parallel circuit includes a corresponding variable capacitor 166 connected in parallel with a variable conductance 168, both connected between node 146 and ground.

The magnitude of both the variable capacitor 166 and the variable conductance 168 vary as a function of the gate voltage V_(GATE) applied to transistor 162. The magnitude of the variable capacitance 166 (indicated as C′) as a function of the gate voltage V_(GATE) applied to transistor 162 is plotted in FIG. 4D. When the gate voltage V_(GATE) is low, the resistance of transistor 162 is high (assuming an N-channel transistor), and not much current flows through the transistor 162. Consequently the capacitance C′ is very low because the circuit 144 a is virtually an open circuit. When the gate voltage V_(GATE) is high, the resistance of the transistor 162 is low, and the capacitor 160 lower terminal is held robustly to ground. Consequently, the capacitance C′ is virtually identical to the magnitude of the capacitor 160 because the circuit 144 a is essentially a capacitor connected between node 146 and ground.

The magnitude of the variable conductance 168 (indicated as g′) as a function of the gate voltage V_(GATE) applied to transistor 162 is plotted in FIG. 4E. When the gate voltage V_(GATE) is low, the resistance of transistor 162, modeled as the variable resistor 164, is high (again assuming an N-channel transistor), and not much current flows through the transistor 162. Consequently the conductance g′ is very low (see, for example, region 170) because the transistor 162 is almost off and the circuit 144 a is virtually an open circuit. When the gate voltage V_(GATE) is high, the resistance of the transistor 162 is low, and the capacitor 160 terminal is held robustly to ground. Consequently, the conductance g′ is very low in this case as well (see, for example, region 172) because the circuit 144 a is essentially a capacitor connected between node 146 and ground. However, at intermediate gate voltages, both the resistor and the capacitor interact and the conductance g′ exhibits a peak (region 174). As long as the VCO circuit (or other circuit to which connected) has enough gain to support the necessary current flow through this conductance, the variable capacitor 144 a circuit may be effectively used to provide a variable amount of capacitance that is controlled by an analog gate voltage V_(GATE), as shown in FIG. 4D.

While a single variable capacitor circuit as shown in FIG. 4A indeed functions as a variable capacitor (i.e., a “varactor”), the performance of such a circuit is affected by certain semiconductor parameters, such as transistor threshold voltage and transistor mobility. Such variations in performance may be reduced, and greater precision and overall linearity of the variable capacitor may be achieved, by using a large group of such circuits connected in parallel, each having a separate capacitor and each controlled by a separate gate voltage control signal. A differential LC tank circuit 180 is shown in FIG. 5 as one possible implementation of the circuit 140 shown earlier in FIG. 3A. As before, an inductor 142 is connected between nodes 146 and 148. Such an inductor may be implemented monolithicly as a spiral inductor, or preferably as a symmetrical series-connected pair of spiral inductors, or even provided off-chip. A variable capacitor 144 (as shown earlier in FIG. 3A) for the tank circuit is implemented by a group of identical variable capacitance circuits 181, 191, 201 in parallel, although it is understood that in actual practice many more than three such circuits are preferably implemented. One such circuit 181 includes a capacitor 184 whose first terminal is coupled to node 146 and whose second terminal is coupled to ground by transistor 186. Circuit 181 also includes a second capacitor 190 whose first terminal is coupled to nod e 148 and whose second terminal is coupled to ground by transistor 188. The respective gate terminal of transistors 186, 188 are both coupled to receive a gate control signal V_(EXP)<0> conveyed on node 182.

Similarly, variable capacitance circuit 191 includes two capacitors 194, 200 having a respective terminal thereof coupled to ground by respective transistors 196, 198 when a gate control signal V_(EXP)<1> conveyed on node 192 is at a suitable voltage (e.g., greater than the transistor threshold voltage), and variable capacitance circuit 201 includes two capacitors 204, 210 having a respective terminal thereof coupled to ground by respective transistors 206, 208 when a gate control signal V_(EXP)<N> conveyed on node 202 is likewise at a suitable voltage.

For this variable capacitance circuit 144, the various gate control signals are provided by a voltage expander 212, which receives the VCO control voltage 118 and generates a plurality of expanded VCO control voltages V_(EXP)<0>, V_(EXP)<1>, . . . V_(EXP)<N> corresponding to the VCO control voltage 118. For example, the expanded VCO control voltages may follow the following relationship:

 V _(EXP) <i>=V _(IN)+(i)(V _(OFFSET))+V _(BIAS)

As this relationship suggests, the V_(OFFSET) and V_(BIAS) values may be chosen to provide a plurality of substantially evenly-spaced output voltage levels that are related to the input voltage V_(IN) (i.e., the VCO control voltage 118). The V_(OFFSET) and V_(BIAS) values may be chosen so that any number of the plurality of substantially evenly-spaced output voltage levels may be greater in voltage than the input voltage V_(IN), with others of the plurality of output voltage levels that are lower in voltage than the input voltage V_(IN). For example, all of the output voltage levels V_(EXP)<i> may be chosen to be greater in voltage than the input voltage V_(IN). The input voltage V_(IN) need not be replicated on one of the plurality of output voltage levels. However, as the voltage expander name somewhat implies (but does not necessarily so require), the V_(OFFSET) and V_(BIAS) values are preferably chosen to provide a plurality of substantially evenly-spaced output voltage levels V_(EXP)<i> that are nominally centered about the input voltage V_(IN).

As a specific example, the voltage expander 212 may be configured to generate nine separate output voltages each 100 mV apart and centered as a group around the input voltage. Assuming that V_(EXP)<0> is the highest voltage and that V_(EXP)<8> is the lowest voltage, the fifth output voltage V_(EXP)<4> is then substantially equal to the input voltage (i.e., the VCO control voltage 118). If, for example, the VCO control voltage 118 at a particular time is equal to 1.8 volts, then the V_(EXP)<i> signals are generated respectively to be 1.4 volts, 1.5 volts, 1.6 volts, 1.7 volts, 1.8 volts, 1.9 volts, 2.0 volts, 2.1 volts, and 2.2 volts. As the VCO control voltage 118 increases or decreases, each of the V_(EXP)<i> output signals increases or decreases in the same direction and by substantially the same amount.

The linear behavior of each V_(EXP)<i> signal may be intentionally or unintentionally limited to an upper and lower bound, as shown in FIG. 6. Each of the exemplary waveforms V_(EXP)<>, V_(EXP)<1>, . . . V_(EXP)<8> is plotted as a function of the input voltage (in this example, the VCO control voltage 118). Each V_(EXP)<i> output signal linearly tracks the input voltage with a corresponding offset voltage. However, the maximum voltage of each V_(EXP)<i> signal is limited to V₂ volts, and the minimum voltage of each V_(EXP)<i> signal is limited to V₁ volts. Such voltage limits may arise from the output range of the voltage expander 212 circuitry and, for example, may be substantially equal to the power supply voltages used in the design of the voltage expander 212 (i.e., VDD and ground). Alternatively, one or more of the voltage limits may be arbitrarily chosen to be a non-power supply value. In an exemplary circuit shown, the voltage limits arise from the head room of the design, and thus the value of V₂ is preferably just below VDD, while the value of V₁ is just above ground. In the variable capacitance circuit 144, the lower limit should preferably be chosen to be lower than the transistor threshold value of the switch transistors such as transistors 186 and 188 (assuming, as shown, N-channel transistors), while the upper limit should preferably be chosen to be higher than the threshold value of the switch transistors by an amount sufficient to substantially turn on the transistors.

Additional details of voltage expanders as used with a plurality of individual variable capacitance circuits in an LC tank circuit for a VCO are described in U.S. Pat. No. 6,137,372 to Welland, which is incorporated herein by reference in its entirety. In particular, the achievement of a greater linear range of capacitance as a function of input control voltage is described in detail therein. Preferred embodiments are described herebelow in the context of an exemplary clock and data recovery application.

The tuning range of such a continuously variable capacitance circuit 144 may be improved by including between nodes 146 and 148 an additional variable capacitance circuit that is adjusted during a calibration routine and generally held staticly thereafter. Referring now to FIG. 7, an exemplary embodiment of an improved tank circuit which includes a second variable capacitance circuit 274 is shown. This circuit is similar to the earlier described variable capacitance circuit 144 except that the various gate control voltages (i.e., in this case, the V_(CAL)<i> voltages) are preferably determined during a power-up calibration routine. Once determined, these gate control voltages V_(CAL)<i> (i.e., gate control signals) are preferably held staticly for as long as the device is powered, or until the range of the variable capacitance circuit 144 in FIG. 5 is exhausted. In such a case the variable capacitance circuit 144 is preferably centered in its dynamic range and a recalibration performed to determine new values for the gate control voltages V_(CAL)<i>.

For clarity, the variable capacitance circuit 274 includes a plurality of individual variable capacitance circuits 364.0, 364.1, . . . 364.N (also referred to as “sub-varactors”), each of which receives a respective one of a corresponding plurality of calibration gate control signals V_(CAL)<0>, V_(CAL)<1>, . . . V_(CAL)<N> from a digital calibration control block 366. Preferably twenty-one (21) such individual variable capacitance circuits are included, although other numbers may be advantageous depending upon the application. As before, each individual variable capacitance circuit 364.i includes a pair of balanced capacitors, each of which is “grounded” through a respective one of a corresponding pair of transistors, both of which transistors being responsive to a common gate control signal. Specifically, each variable capacitance circuit 364.i includes a capacitor 360.i having a first terminal connected to node 146 and having a second terminal coupled to ground through a transistor 362.i controlled by a corresponding calibration gate control signal V_(CAL)<i>, and further includes a capacitor 361.i having a first terminal connected to node 148 and having a second terminal coupled to ground through a transistor 363.i also controlled by the corresponding calibration gate control signal V_(CAL)<i>.

Each of the calibration gate control signals V_(CAL)<i> are generated by a digital calibration control block 366 which determines, preferably at least during a power-up calibration routine, which signals to drive high and which to drive low. Once appropriately driven, the various V_(CAL)<i> signals preferably remain unchanged for as long as the device remains powered, or alternately until the dynamic tuning range is exceeded. The various V_(CAL)<i> calibration gate control signals are usually only changed during an initial calibration, and particularly not while the phase locked loop 130 remains operational and within specification. Consequently, each V_(CAL)<i> signal may be driven between full logic levels with little regard to ramp rate or transition time, and more than one such V_(CAL)<i> signal may be driven (i.e., and change logic states) at the same time.

The digital calibration control block 366 receives various calibration control signals conveyed on node(s) 367 to increment or decrement the total number of V_(CAL)<i> signals that are driven high, and thus increment or decrement the total effective capacitance “between” nodes 146 and 148. Such circuitry depends upon the particular calibration scheme employed by the device, is non-critical in performance, and its design is well within the capabilities of one skilled in the art.

Referring now to FIG. 8, the dynamic tuning range of the LC tank circuit may be increased by including yet another group of variable capacitance circuits 299 whose gates are generally held staticly during operation, but which may be slowly switched on or off to gradually vary the amount of capacitance on nodes 146 and 148. Like the above described variable capacitance circuit 274, a plurality of individual variable capacitance circuits 304.0, 304.1, . . . 304.N are provided, each connected between nodes 146 and 148, and each of which receives a respective one of a corresponding plurality of slow digital gate control signals V_(SD)<0>, V_(SD)<1>, . . . V_(SD)<N> conveyed respectively on nodes 305.0, 305.1, . . . 305.N. The variable capacitance circuit 299 preferably includes thirty-six (36) such individual variable capacitance circuits 304.i, although other choices may be advantageous depending upon the application. As before, each variable capacitance circuit 304.i includes a capacitor 300.i having a first terminal connected to node 146 and having a second terminal coupled to ground through a transistor 302.i controlled by a corresponding gate control signal V_(SD)<i>, and further includes a capacitor 301.i having a first terminal connected to node 148 and having a second terminal coupled to ground through a transistor 303.i also controlled by the corresponding slow digital gate control signal V_(SD)<i>.

A slow digital control block 306 receives a capacitance control signal conveyed on node 214 and accordingly generates the plurality of slow digital control signals V_(SD)<0>, V_(SD)<1>, . . . V_(SD)<N>. It is preferable to switch only one slow digital control signal V_(SD)<i> at a time, with all remaining other slow digital control signals V_(SD)<i> held at either a logic low or a logic high value, so that their corresponding capacitors are either essentially open-circuited or are generally well-grounded and therefore electrically “connected” to nodes 146, 148. By switching only one such slow digital control signal V_(SD)<i> at a time, then only one such pair of switching transistors (e.g., transistors 302.0, 303.0) is partially turned on at a time, and the resulting conductance peak described earlier (i.e., in respect to FIG. 4E) only occurs in a single transistor pair during its turn-on or turn-off transient. Moreover, the slow digital control circuit 306 is preferably configured to provide a very slow rise time and fall time on whichever slow digital control signal V_(SD)<i> changes logic state at any given time, while holding all other slow digital control signals quiescent at one of the static levels. This allows the value of the total capacitance to change much more slowly and reduces perturbations to the overall system.

Referring now to FIG. 9, a gate control circuit 220 is shown which is an exemplary embodiment of the slow digital control circuit 306 and which accomplishes slow rise and fall times on its various gate control output signals. A digital control circuit 222 receives the capacitance control signal 214 and generates each of the “slow digital” gate control signals V_(SD)<0>, V_(SD)<1>, . . . V_(SD)<N> so that at most only one changes state at a time. Each output 223, 225, . . . , 227 of the digital control circuit 222 is buffered by a respective “starved” inverter 224, 226, . . . , 228 which is intentionally undersized relative to its load capacitance so that its output rise and fall time is artificially slowed down compared to most other circuits. To achieve even slower transition times of the slow digital control signals, additional load capacitance, such as load capacitor 230, may be included on the various control signal output nodes, such as node 305.N. These load capacitors may be implemented on-chip or alternatively be provided external to the integrated circuit to allow larger capacitor sizes than is practical with numerous on-chip load capacitors.

Referring now to FIG. 10, a switch control circuit 240 is shown which is an exemplary embodiment of the slow digital control circuit 306 and which receives one or more capacitance control signal(s) 214 and generates accordingly a plurality of slow digital switch control signals. Moreover, the switch control circuit 240 accomplishes slow rise and fall times on its various control signal outputs without potentially utilizing a separate load capacitor on each control signal output. A digital control circuit 242 receives the capacitance control signal(s) 214 and generates the quiescent value for each switch control signal. A slow transition generator 244 is provided to generate on its output node 245 a signal having a slow risetime and a slow falltime which is shared as needed to generate each switch control signal. To cause a particular switch control signal V_(SD)<i> to change states from an “old value” to a “new value,” the slow transition generator 244 output is first set to the “old value.” The particular switch control signal node is disconnected from the corresponding output of the digital control circuit 242 and connected to the output of the slow transition generator 244. The slow transition generator is then commanded to transition its output from the “old value” to the “new value,” and the corresponding output of the digital control circuit 242 is brought to the “new value” preferably during this transition. Alternatively, the corresponding output of the digital control circuit 242 may be brought to the “new value” after the transition. The particular switch control signal node is then disconnected from the output of the slow transition generator 244 and then reconnected to the corresponding output of the digital control circuit 242 to maintain the “new value.” Since preferably only one such switch control signal is changed at a time, a single slow transition generator 244 may be shared without contention. Alternatively, each output of the digital control circuit 242 could also be provided with a corresponding slow transition generator, or a smaller group of slow transition generators could be shared among a larger number of potential outputs.

Looking in detail again at FIG. 10, the switch control circuit 240 generates a plurality of slow digital control signals V_(SD)<0>, V_(SD)<1>, V_(SD)<2>, . . . V_(SD)<N>. There can be any of a variety of choices for the exact number of such gate control signals, which may be collectively referred to as V_(SD)<N:0> or alternatively as the group of signals V_(SD)<i>. The quiescent value of each slow digital control signal V_(SD)<0>, V_(SD)<1>, V_(SD)<2>, . . . V_(SD)<N> is generated by the digital control circuit 242 on a corresponding one of a plurality of its outputs VALUE<0>, VALUE <1>, VALUE<2>, . . . VALUE<N>. Normally each slow digital control signal V_(SD)<i> is connected through a corresponding switch 250.i to a corresponding VALUE<i> output signal of the digital control circuit 242. The digital control circuit 242 also generates a plurality of select signals SEL<N:0> to control which slow digital control signal V_(SD)<i> is connected to the output of the slow transition generator 244. Preferably at most one of the SEL<i> signals is active to connect the corresponding slow digital control signal V_(SD)<i> to the output of the slow transition generator 244 (i.e., preferably using a “one-hot” coding), with each remaining SEL<i> signal being inactive to connect the corresponding slow digital control signal V_(SD)<i> to the corresponding VALUE<i> output signal. Quiescently, preferably none of the SEL<i> signals are active, and consequently each slow digital control signal V_(SD)<i> is connected to the corresponding VALUE<i> output signal of the digital control circuit 242. Lastly, at least one transition control signal is generated by the digital control circuit 242 and conveyed on node(s) 246 to the slow transition generator 244 to control the level (and possibly the rise and/or fall time) of the slow transition output signal conveyed on node 245. As configured herein, each respective switch 250.i functions in effect as a multiplexer circuit to connect the respective V_(SD)<i> signal to either the respective VALUE<i> signal or to the shared slow transition signal on node 245.

The slow transition generator 244 may be implemented using any of a variety of suitable circuits, including a starved inverter (with or without a separate load capacitor, which could even be an off-chip load capacitor) as described above. In such a case the transition control signal conveyed on node 246 may be a voltage level signal carried on a single node. A preferable implementation of the slow transition generator 244 includes a digital-to-analog converter circuit (DAC) which receives a digital control word from the digital control circuit 242 and generates an analog voltage accordingly. In this case the transition control signal is preferably an M-bit digital word and the node 246 is preferably an M-bit bus rather than a “single wire”. The transition of such a DAC's output signal from an “old value” to the “new value” may be accomplished arbitrarily slowly by sequencing the digital control word through all the intermediate values between those corresponding to the “old” and “new” values. Such sequencing may be accomplished at any arbitrarily slow rate since it is controlled by the digital control circuit 242. In such a configuration, the output of the DAC follows a stair-step pattern. However, if the precision of the DAC is sufficiently high, the magnitude of the individual “steps” may be quite small, and the resulting waveform may generally be a reasonably linear and arbitrarily long “slow ramp.” Thus, as used herein, the output signal of a slow transition generator or other “ramp generator” need not be a true linear ramp in a literal sense, but rather a signal having a controlled slow transition.

As a detailed example for additional clarity, suppose that the capacitance control signal conveyed to the digital control circuit 242 on node 214 calls for an increase in the capacitance between nodes 146 and 148. Assume that the slow digital control signal V_(SD)<1> conveyed on node 305.1 is initially at a low voltage, and the associated switch transistor (e.g., transistors 302.1 and 303.1) are turned off. Further assume that the digital control circuit 242 determines that the desired increase in total capacitance is to be accomplished by transitioning the slow digital control signal V_(SD)<1> to a high voltage level, which serves to turn on the associated switch transistors 302.1 and 303.1. The slow digital control signals V_(SD)<i> are preferably full CMOS level signals with a low level equal to ground and a high level equal to VDD (e.g., 2.5 V), while the switch transistors 302.1 and 303.1 are preferably enhancement mode FETs, which for this instructive example will also be assumed.

If the slow transition generator output on node 245 is not already low, the transition control signal on node 246 is first driven appropriately to cause the slow transition generator 244 to drive its output node 245 to ground. Then the SEL<1> signal is driven active to disconnect the slow digital control signal V_(SD)<1> from the VALUE<1> output signal, and to connect the V_(SD)<1> signal to the output of the slow transition generator 244 (as shown in FIG. 10). Each remaining SEL<i> signal remains inactive to connect the corresponding V_(SD)<i> signal to the corresponding VALUE<i> output signal. Next, the transition control signal on node 246 is driven appropriately to cause the slow transition generator 244 to drive its output node 245 to VDD (with a controlled and preferably very slow rise time, of course). At preferably about the same time, the VALUE<1> output signal is driven from ground to VDD, which may occur with a normal rise time. Lastly, the SEL<1> signal is driven inactive to disconnect the V_(SD)<1> signal from the output of the slow transition generator 244 and reconnect it to the VALUE<1> output signal, which maintains the VDD level on the V_(SD)<1> signal and allows the slow transition generator 244 to be used for “transitioning” another slow digital control signal when needed. The particular VALUE<1> output signal may be driven (in this example from ground to VDD) at any convenient time whenever the V_(SD)<1> signal is disconnected from the VALUE<1> output signal, which may be prior to, during, or after the slow transition generator output signal on node 245 is caused to transition with its slow ramp.

One skilled in the art may appreciate a wide variety of specific configurations and sequences which may be advantageously utilized with the basic structure shown in FIG. 10. For example, the slow digital control signal V_(SD)<1> may remain connected to the output of the slow transition generator 244 after its output transition is complete without reconnecting the V_(SD)<1> signal to the VALUE<1> output signal. When the slow transition generator is needed again, the V_(SD)<1> signal may be then reconnected to the VALUE<1> output signal, and the slow transition generator made available to drive another slow digital control signal. However, if power dissipation is a concern, it may be preferable to disconnect the slow transition generator 244 from the driven signal once its transition is complete, as described above, and power-down the entire slow transition generator 244 until needed again.

Each of the switches 250.i (collectively referring to switches 250.0, 250.1, . . . through 250.N) may be implemented easily by using a full CMOS transfer gate structure (an NMOS FET and a PMOS FET driven with complementary signals) or an equivalent thereto. Alternatively, a single polarity transfer gate (either NMOS or PMOS) may also be used if the particular static voltages chosen for the low level and high level, relative to VDD and ground, may be communicated adequately. Other circuits may conceivably also be used, as the performance of such switch circuits are not critical and there is negligible current flow through the switches. Suitable implementations of the digital control circuit 242 and the slow transition generator 244 may be straightforwardly carried out by those skilled in the art based on the teaching herein.

A switch control circuit similar to the above-described switch control circuit 240 may be advantageously incorporated within the digital control block 132 of the phase locked loop 130 (as depicted in FIG. 2). Referring now to FIG. 11, an exemplary embodiment is shown of a portion of the phase locked loop 130 earlier described in relation to FIG. 2. A portion of the VCO 136 is shown in detail which includes a symmetrical pair of series connected inductors 142A and 142B, and three variable capacitance circuits 270, 272, and 274, each coupled between nodes 146 and 148 to form a tunable LC tank circuit for the remainder of the VCO 136. Since an on-chip inductor is typically implemented as a spiral inductor (having an inner terminal and an outer terminal), two symmetrical inductors are preferably implemented in series to achieve a more balanced circuit. These three variable capacitance circuits 270, 272, and 274 (as well as stray capacitances) together determine the total capacitance of nodes 146 and 148 and thus collectively determine the frequency of the VCO 136. The first of these variable capacitance circuits, labeled 270, is controlled by the VCO control voltage signal 118 and is the primary tunable circuit element for normal operation of the phase locked loop. Such a variable capacitance circuit 270 is preferably implemented generally as described in regards to FIG. 5, and more preferably as described below.

The second of the variable capacitance circuits, collectively labeled as 272, receives a plurality of dynamically changeable slow digital gate control signals V_(SD)<0>, V_(SD)<1>, . . . V_(SD)<35> from the digital control block 132 which collectively adjust the total capacitance of (i.e., “between”) nodes 146 and 148 so that the VCO control voltage 118 dynamically operates at a nominal voltage reasonably centered within its available voltage range. This second variable capacitance circuit 272 is preferably used to compensate for environmental changes (e.g., temperature and/or voltage changes) and parametric changes (e.g., component parameters, such as threshold voltage) that may occur during operation and particularly well after any power-up calibration. Such a variable capacitance circuit 272 is preferably implemented generally as described in regards to FIG. 8 through FIG. 10, and more preferably as described in regards to FIG. 11 and as further described below.

The third of the variable capacitance circuits, labeled 274, is preferably adjusted during a calibration routine at power-up, after reset, or if the slow switched variable capacitance circuit 272 is out of range, to a static value that nominally allows the VCO control voltage 118 to operate reasonably centered within its available voltage range. This third variable capacitance circuit 274 is preferably used to compensate for normal manufacturing variations in semiconductor parameters as fabricated, as well as to compensate for the initially-encountered values of the environmental variables during each powering-up of the device. Such a variable capacitance circuit 274 is preferably implemented generally as described in regards to FIG. 7, and more preferably as described below.

In operation, the digital control block 132 senses the VCO control voltage 118 to determine whether it remains within a predetermined portion of its operating range preferably somewhat centered within its operating range). Whenever the VCO control voltage 118 falls outside this range, the digital control block 132 adjusts very, very slowly the value of the variable capacitance circuit 272. Absent the phase locked loop 130, such a change would result in a change of VCO frequency. But to remain in phase and frequency lock, the phase locked loop 130 responds to the changing value of the variable capacitance circuit 272 by adjusting the VCO control voltage 118 by an amount which results in an equal but offsetting change in capacitance of the variable capacitance circuit 270. Consequently, the VCO frequency remains unchanged. The digital control block 132 preferably adjusts the value of the variable capacitance circuit 272 either upwards or downwards, as appropriate, to “drive” the VCO control voltage 118 toward the center of its operating range. For example, assume that a higher VCO control voltage 118 corresponds to a higher value of the variable capacitance circuit 270 (i.e., compared to lower values of the VCO control voltage 118). If the digital control block 132 senses that the VCO control voltage 118 is too high, one or more of the gate control signals V_(SD)<i> are preferably driven so as to increase the value of variable capacitance circuit 272. The phase locked loop 130 responds by driving the VCO control voltage 118 downward by an amount which results in a decrease in value of the variable capacitance circuit 270 by an amount equal to the increase in value of the variable capacitance circuit 272. The VCO frequency thus remains unchanged, but the VCO control voltage 118 has been driven toward the center of its operating range. By so doing, the phase locked loop operation is improved because the VCO control voltage operates nearer to the center of its range, and in particular, never operates close to either extreme of its range, and consequently improves performance by reducing sensitivity to the voltage on node 118.

With this understanding presented thus far, the operation of the circuitry depicted in FIG. 11 should be readily apparent to one skilled in the art. Nonetheless, several details warrant discussion. The second of the variable capacitance circuits, collectively labeled as 272, is formed of a plurality of individual variable capacitance circuits 272.0, 272.1, 272.2, . . . 272.35, each of which receives a respective one of a corresponding plurality of dynamically changeable slow digital gate control signals V_(SD)<0>, V_(SD)<1>, V_(SD)<35> from the digital control block 132. Thirty-six such individual variable capacitance circuits (as shown) is a particularly advantageous number, although other choices may be equally advantageous depending upon the application.

In this embodiment a DAC 268 is used to generate a controlled slow rise and fall time (both high-going and low-going) as described above. A 15-bit control word is generated by the digital control block 266 to communicate the desired value of the DAC's output signal conveyed on node 245 to each of the individual switches 250.i. Such a 15-bit DAC provides 32768 individual voltage levels that may be generated on its output, which allows its output waveform to be generated with an arbitrarily long transition time, yet one having relatively small “stair-steps” and thus relatively smooth. For example, if the full-scale transition of the DAC 268 is from 0.0 to 2.5 volts, the magnitude of each stair step is approximately 76 μV. Such transitions on node 245 may be generated as linear voltage ramps, as well as more complex functions of voltage vs. time. Although a DAC having 15-bit resolution is incorporated in this embodiment, other resolutions may also be used. For example, a DAC having 10-bit resolution may provide sufficiently small granularity to achieve adequate performance in many applications. Other resolutions, such as 4 bits, 6 bits, or 8 bits, may also be chosen.

A pair of comparators 262, 264 monitor the voltage of the VCO control voltage 118 to determine whether it is too high or low in magnitude. Comparator 262 compares the VCO control voltage 118 to an upper voltage limit signal conveyed on node 263 and generates an OVERFLOW signal on node 276 which is communicated to the digital control block 266. Comparator 264 compares the VCO control voltage 118 to a lower voltage limit signal conveyed on node 265 and generates an UNDERFLOW signal on node 277 which is likewise communicated to the digital control block 266. Comparators 262 and 264 are each preferably implemented having 50-200 mV of hysteresis to prevent its respective output from chattering when its input is near its threshold voltage, and more preferably each have about 100 mV of hysteresis.

Using any of a variety of well-known and not particularly critical design techniques, the digital control block 266 generates the DAC control word DAC<14:0>, the element value signals VALUE<35:0>, and the switch selection signals SELECT<35:0>, preferably in a fashion to behave consistently with the above description. For example, the element value signals VALUE<35:0> are preferably driven with a “thermometer code” in which each additionally driven-high signal is adjacent to the most recently driven-high signal. Nonetheless, such coding is not required, and any coding in which additional or lesser numbers of the element value signals are driven high or low to accomplish a desired change in capacitance of the variable capacitance circuit 272, as described above, is equally satisfactory. The switch select signals SELECT<35:0> are preferably driven with a “one-hot” coding so that, at most, only one individual variable capacitance circuit 272.i is connected to the DAC 268 and is potentially changing in value at any given time. However, other schemes are possible. For example, two or more such individual variable capacitance circuits 272.i could be driven at the same time with a gate control voltage having a slowly-changing voltage magnitude, although the perturbation to the VCO 136 may be more significant. The digital control block 266 also preferably generates a RECALIBRATE signal which is asserted when an UNDERFLOW or OVERFLOW signal is generated which attempts to adjust the slow digital capacitance circuit 272 beyond its control range. A recalibration of the DC calibration capacitance circuit 274 may then be performed to re-center the control voltage 118 and the slow-switched variable capacitance circuit 272.

The digital control block 266 receives a RAMP RATE CONTROL signal conveyed on node 267 to provide for different ramp rates (i.e., transition times) on the DAC output node 245. When the phase locked loop 130 is configured for a clock and data recovery application, the transition time is advantageously varied as a linear function of bit data rate. For example, for a 2.4 Gbit/second data rate (OC-48), a full-scale transition time on node 245 of 164 milliseconds is particularly desirable to allow the phase locked loop 130 to maintain lock during the ramp time, while for a much slower 155 Mbit/second data rate (OC-3), a transition time of 2.62 seconds is particularly desirable. As described above, with sufficient precision, the DAC 268 can easily generate such a slow ramp time, yet still produce an output signal that approximates a relatively smooth, linear voltage waveform. At such slow ramp times, of particular advantage is the ability of the phase locked loop 130 to maintain performance within virtually identical specifications as if no variable capacitance circuit 272.i was being changed. For example, such a phase locked loop 130 may achieve jitter tolerance and jitter generation performance during a transition which is substantially equal to its steady-state performance. Said differently, long term parametric drifts, slow temperature changes, and slow voltage changes may be compensated without negatively impacting the jitter tolerance, jitter generation, or other subtle performance specifications of the phase locked loop 130. Preferably, the transition time is chosen to balance the competing desires of being fast enough to adequately track anticipated environmental changes, yet being slow enough to preserve performance specifications, such as jitter tolerance and jitter generation, during the transition. With the relatively slow rate of environmental changes likely to be encountered in, for example, a SONET application, the transition time may preferably be chosen to be much slower than that believed merely adequate to preserve performance specifications, and still be fast enough for the anticipated rate of environmental changes.

Referring now to FIG. 12, a preferred embodiment of a single stage 272.i within the “slow digital” variable capacitance circuit 272 is shown. A pair of P-channel precharge transistors 284 and 285 are included so that when the slow digital gate control signal V_(SD)<i> is low, the intermediate nodes 286 and 287 are respectively coupled to nodes 146 and 148. This prevents the voltage of such intermediate nodes 286 and 287 from slowly discharging or otherwise floating, which could cause unwanted noise on the LC tank circuit nodes 146 and 148 and injection from the. intermediate nodes to the local substrate. The precharge transistors 284 and 285 are both gated by a common PRE<i> signal whose low level is preferably ground and whose high level is an internally regulated value below VDD so that the respective gate terminals of transistors 284 and 285 are not driven too much higher than the respective source terminals (i.e., nodes 146 and 148). These PRE<i> signals are preferably also generated by the digital control block 266 (connection not shown). Such technique is described and explained in greater detail in U.S. Pat. No. 6,147,567 to Welland, et al, which is incorporated herein by reference in its entirety.

An exemplary relative timing of the PRE<i> signal and V_(SD)<i> signal for a given stage 272.i are depicted in FIG. 13. When the V_(SD)<i> signal is low, the PRE<i> signal is also low. Before the V_(SD)<i> signal is transitioned from low to high, the PRE<i> signal is first brought high to turn off the precharge transistors 284.i and 285.i. Conversely, after the V_(SD)<i> is brought low, the PRE<i> signal is also brought low to respectively maintain the voltage of the intermediate nodes 286.i and 287.i at the voltage of tank nodes 146 and 148.

Referring now to FIG. 14, a portion of a preferred embodiment of the VCO 136 is depicted. A pair of cross-coupled CMOS inverters 373 and 374 are coupled to the tank nodes 146 and 148 and provide gain to the resonant LC tank circuit described above. A voltage regulator 371 is provided to generate a regulated VDD for the VCO to reduce the effects of noise on VDD as well as reduce the variation of nominal VCO operating frequency caused by variations in the VDD level. A differential clock buffer 375 receives the signals on the differential tank nodes 146 and 148 and generates a buffered pair of differential VCO CLK signals labeled VCO_CLK and VCO_CLK#.

Referring now to FIG. 15, a preferred embodiment of a single stage 274.i within the DC calibration variable capacitance circuit 274 is shown. A pair of P-channel “precharge” transistors 364 and 365 are turned on when the calibration control signal V_(CAL)<i> is low so that the intermediate nodes 366 and 367 are respectively coupled to nodes 146 and 148. This prevents the voltage of such intermediate nodes 366 and 367 from slowly discharging or otherwise floating, which could cause unwanted noise on the LC tank circuit nodes 146 and 148 and injection from the intermediate nodes to the local substrate. The precharge transistors 364 and 365 are both gated by a common P_(CAL)<i> signal whose low level is preferably ground and whose high level is an internally regulated value below VDD so that the respective gate terminals of transistors 364 and 365 are not driven too much higher than the respective source terminals (i.e., nodes 146 and 148).

The relative timing of the P_(CAL)<i> signal and V_(CAL)<i> signal for an individual variable capacitance circuit stage 274.i are not particularly critical. They can be switched substantially together, or alternatively may be sequenced so that before the V_(CAL)<i> signal is switched from low to high, the P_(CAL)<i> signal is first brought high to turn off the precharge transistors 364.i and 365.i. Conversely, the P_(CAL)<i> signal may be brought low after the V_(CAL)<i> signal is low enough to turn off the transistors 362.i and 363.i so that unnecessary current does not flow from nodes 146 and 148 to ground. Additional details of such calibration techniques and circuits are described and explained in U.S. Pat. No. 6,147,567 which is incorporated herein by reference in its entirety.

The various individual variable capacitance circuits 274.i may be sized identically, or may be individually weighted to achieve a range of control using fewer individual stages. To reduce calibration time, a particularly desirable configuration includes three groups of capacitance circuits weighted to achieve a course, medium, and fine tuning control during the DC calibration. The fine control section preferably includes six such variable capacitance circuits 274.i which are preferably evenly weighted, and each sized to preferably cause about a 0.125% change in VCO frequency when one such circuit is switched in or out. The medium control section preferably includes eight such variable capacitance circuits 274.i which are preferably evenly weighted and sized to be four times larger than each of the fine control circuits. The course control section preferably includes seven such variable capacitance circuits 274.i which are preferably evenly weighted and sized to be eight times larger than each of the medium control stages. Alternatively, the variable capacitance circuits 274.i need not be evenly weighted, but may be weighted using a radix of two, or alternatively a radix somewhat less than two.

The calibration routine preferably first adjusts the course array, switching one element at a time in a “thermometer code” fashion until the VCO is close to the desired frequency. Then the medium section is adjusted in the same thermometer code fashion to get closer to the desired frequency. Lastly, the fine control stages are adjusted in the same thermometer code fashion to achieve an even finer frequency resolution and get even closer to the desired frequency. The tuning range overlap of the fine control section over the medium control section, and the medium control section over the course control section provides that even with a mismatch of certain stages, no large frequency gap is present over which the calibration routine cannot reach.

As depicted in the variable capacitance circuit 274.i, a lateral transistor 368 may be included to connect the intermediate node 366 between capacitor 360 and transistor 362 to the intermediate node 367 between capacitor 361 and transistor 363. This additional transistor 368 lowers the impedance of the grounding path and is preferably used in the larger such circuits, preferably only in the course and medium control circuits. Such a technique is more fully explained in the above-referenced U.S. Pat. No. 6,147,567 to Welland, et al.

Referring now to FIG. 16, a preferred voltage expander 212 is illustrated which generates twenty-one expanded voltage V_(EXP)<i> signals, one of which having a voltage substantially equal to the input voltage, with one half of the remaining V_(EXP)<i> signals higher in voltage than the input voltage, and the other half of the remaining V_(EXP)<i> signals lower in voltage than the input voltage. A buffer 322 receives the VCO control voltage 118 and generates on its output node 324 the V_(EXP)<10> signal preferably having substantially the same voltage as its input voltage. The buffer 322 preferably drives its output node 324 with a relatively low output impedance, for reasons described below.

Assuming briefly that node 324 is held robustly at its desired voltage (e.g., equal to the VCO control voltage), the first one of the V_(EXP)<i> voltage signals, namely V_(EXP)<0>, is generated by passing a controlled current from current source 326 through a resistor 327 whose other terminal is anchored at the voltage of node 324. The desired voltage offset between node 324 and the V_(EXP)<0> signal may be accomplished by adjusting the magnitude of either or both of the current source 326 or the resistor 327. Each of the respective remaining V_(EXP)<i> voltage signals is generated by passing a controlled current from a respective current source through a respective resistor having a predetermined value which is connected to node 324.

In the embodiment shown, half of the V_(EXP)<i> signals (i.e., V_(EXP)<0> through V_(EXP)<9>) have a voltage higher than the voltage of node 324, while the other half of the V_(EXP)<i> signals (i.e., V_(EXP)<11> through V_(EXP)<20>) have a voltage lower than the voltage of node 324. One particular advantage of this arrangement is the balancing of current flow into node 324 with the current flow from node 324. For example, if the magnitude of current source 326 is identical to that of current source 334, then no net current flows into or from node 324. The buffer 322 functions to provide the voltage on node 324, but the output impedance of buffer 322 need not absorb any net current into node 324. Similarly, the aggregate current flow of current sources 326, 328, . . . , 330, and 332 is preferably substantially identical to the aggregate current flow of current sources 334, 336, . . . , 338, and 340. Consequently, the net current flow into node 324 is essentially zero and the output impedance demands of buffer 322 are reduced substantially. In the preferred embodiment shown, the value of each current source is preferably 50 microamps, and each resistor shown is a multiple of a unit resistor R having a value equal to 2.7 kOhms. For example, resistor 333 has a preferred value of 2.7 kOhms, while resistor 327 has a preferred value of 27 kOhms. As a result, the voltage difference between each adjacent expanded voltage V_(EXP)<I> is preferably 135 mV. Referring briefly back to FIG. 6, the upper limit (V₂) and lower limit (V₁) of the twenty-one expanded voltage signals V_(EXP)<i> are preferably 2.1 and 0.3 volts, respectively, which may be accomplished using traditional current source (current mirror) structures and supply voltages of, for example, 2.5 volts and ground.

Referring now to FIG. 17, an exemplary embodiment of an individual stage 270.i within the continuously variable capacitance circuit 270 (i.e., the PLL “tuning” capacitance circuit) is shown. This circuit is similar to the earlier described variable capacitance circuit 144 having a pair of capacitors 380 and 381 respectively coupled to tank nodes 146 and 148, and which are coupled to ground through respective transistors 382 and 383. The preferred circuit stage also includes a pair of transistors 390 and 391 having a low-level DC bias voltage V_(BIAS) impressed upon their gate terminals, each of which acts as a high-valued resistor connected to the respective intermediate node 386 and 387. The V_(BIAS) voltage is preferably generated by a current source 393 flowing through a diode-connected transistor 394 and communicated to each individual stage. Moreover, in those stages whose capacitors 380 and 381 are very small, another pair of fixed capacitors 388 and 389 are connected respectively between the respective intermediate node and ground. Such circuits are described in additional detail in U.S. Pat. No. 6,147,567 which is incorporated herein by reference in its entirety.

The preferred circuit stage depicted also includes a second pair of transistors 384 and 385, and receives a pair of expanded voltage signals V_(EXP)A and V_(EXP)B rather than just one such expanded voltage signal. For the nineteen of the preferred twenty-one total circuit stages, two expanded voltage signals V_(EXP)<i> and V_(EXP)<i+2> (i.e., separated by one expanded voltage signal) are received by the stage. For example, one such stage receives V_(EXP)<2> and V_(EXP)<4>, the next stage receives V_(EXP)<3> and V_(EXP)<5>, and so forth. The two end-most stages receive two adjacent expanded voltage signals (V_(EXP)<0> and V_(EXP)<1> for one, and V_(EXP)<19> and V_(EXP)<20> for the other). By including a transistor responsive to each of two such expanded voltage signals, a greater linearity is achieved for a given variable capacitance stage, and consequently there is a greater linearity of the entire circuit than would be otherwise expected for a given number of individual stages (here shown as twenty-one such stages).

Referring now to FIG. 18, another embodiment of the earlier-described circuit of FIG. 11 is shown which depicts a preferred embodiment of the continuously-variable capacitance circuit 270 and other details of related portions of the circuit. In particular, the voltage expander 212 is shown receiving the VCO control voltage 118 and generating a group of twenty-one expanded voltage signals V_(EXP)<20:0> which are conveyed in pairs to individual variable capacitance circuit stages 270.i as described above. As before, a pair of comparators 262, 264 essentially monitor the voltage of the VCO control voltage 118 to determine whether it is too high or low in magnitude. Specifically, the comparator 262 preferably compares a particular expanded voltage signal V_(EXP)<Y> to the upper voltage limit signal conveyed on node 263 and generates an OVERFLOW signal on node 276 which is communicated to the digital control block 266. The comparator 264 preferably compares a second particular expanded voltage signal V_(EXP)<i> to the lower voltage limit signal conveyed on node 265 and generates an UNDERFLOW signal on node 277 which is likewise communicated to the digital control block 266. Comparators 262 and 264 are each preferably implemented having 50-200 mV of hysteresis to prevent its respective output from chattering when its input is near its threshold voltage, and more preferably each have about 100 mV of hysteresis. Comparing a pair of expanded voltage signals V_(EXP)<i> to infer the magnitude of the VCO control voltage 118 rather than comparing the VCO control voltage itself allows level shifting to better accommodate the desired input level for the comparators 262 and 264 and to ensure sufficient head-room for proper operation of each comparator, even for wide voltage excursions of the VCO control voltage 118.

Referring now to FIG. 19, a block diagram is shown of a preferred clock and data recovery circuit utilizing the slow digital calibration of the VCO frequency as described above. Such a circuit is preferably calibrated at startup (or at a recalibration) to 2.5 GHz, +/−0.125% from a nominal frequency range as wide as 2.5 GHz +/−20% as fabricated. The slow digital switching capability is preferably arranged to provide a tracking range up to a +/−2.5% variation in frequency, to track aging of components and temperature and voltage variations over time. The tuning varactor within the PLL itself is preferably configured to provide a +/−0.5% tuning range over a typical varactor control voltage range from 0.5 to 1.5 volts.

The teachings described thus far may be utilized in a wide variety of circuits. For example, the slow digital switching of a plurality of variable capacitance circuits to achieve a very slowly varying capacitance between two nodes, as described above, may be extended to a plurality of variable resistor circuits to achieve a slowly varying resistance between two nodes.

Referring now to FIG. 20, an input impedance circuit 400 is shown which utilizes a slow-switching digital feedback path to constantly adjust the impedance of an on-chip input termination resistor without abrupt changes in its magnitude. If incorporated within a clock and data recovery circuit (CDR circuit), such abrupt changes in the input resistance could cause decreased jitter tolerance or worse, outright data errors. A pair of transmission lines 402 (i.e., having a controlled impedance) is shown communicating a positive data input signal DIN+ onto node 404 and communicating a negative data input signal DIN− onto node 406, which DIN+ and DIN− signals together form a differential input signal DIN. A current source 408 and resistor 409 together provide a bias for node 404, and a current source 410 and resistor 411 together provide a bias for node 406. An input termination resistor 412 is provided on-chip between the two input nodes 404 and 406, and preferably should always maintain a magnitude equal to twice that of the impedance of each transmission line 402.

The input termination resistor 412 includes two fixed resistors 430, 431 and a plurality of individual variable resistance circuits, each of which includes a first resistor, a transistor, and a second resistor connected together in series between nodes 404 and 406. The first such variable resistance circuit includes a transistor 414 (shown, for this embodiment, as a P-channel transistor) and two resistors 413, 415. When transistor 414 is conductive, a series circuit is formed between nodes 404 and 406 whose value is the sum of the two resistors 413, 415 and the channel resistance of transistor 414. While the resistance of resistors 413 and 415 is assumed to be relatively constant, the resistance of the transistor 414 channel may be varied by changing its gate voltage V_(A) conveyed on node 435. When non-conductive (i.e., “off”), the resistance of transistor 414 is very high, and the series circuit degenerates to a virtual open circuit. Consequently the total resistance of the series circuit may be controlled by varying the gate voltage on node 435 to achieve a total series resistance which varies between an upper limit equal to an open circuit and a lower limit approaching the sum of the two resistors (assuming the transistor is large enough relative to the size of the two resistors). Lower values of the gate voltage V_(A) conveyed on node 435 result in a lower resistance of (P-channel) transistor 414. Advantageously, when two fixed resistors are included in series with a transistor, some of the signal voltage swing at the input is dropped across the fixed resistors. Consequently, there is less of a voltage drop across the channel of the transistor, and the resistance between input terminals is closer to the desired value (largely determined by the fixed resistors). Alternatively, if the magnitude of the input signal is limited, a single transistor may be used for the variable resistor and the two fixed resistors omitted.

The remaining ones of the plurality of individual variable resistance circuits each includes a first resistor 417.i, a transistor 418.i, and a second resistor 419.i connected in series between nodes 404 and 406, which transistor 418.i is controlled by a gate control signal conveyed on node 438.i. While three such circuits are actually depicted in the figure, their labeling (e.g., transistors 418.0, 418.1, . . . 418.N) clearly indicate that N+1 such circuits are contemplated. The contribution of each such variable resistance circuit, along with fixed resistors 430 and 431, give rise to the total resistance of the input termination resistor 412. As will become clear in the following description, in this embodiment the various gate control signals are accordingly driven to continuously keep the magnitude of the input termination resistor 412 equal to a precise ratio of an external precision resistor. Exemplary values of N are also described herebelow.

The input impedance circuit 400 includes an I_(TUNE) generator 460 which includes an external resistor 462, preferably a precision resistor, having a value equal to R_(EXT). A transistor 472 receives a gate bias voltage V_(B) conveyed on node 468 and functions as a current source to cause a reference current I_(REF) to flow through the external resistor 462, thus causing a reference voltage V_(REF1) to be established at node 463 and communicated to the inverting input of operational amplifier 466. An internal resistor 464 is implemented having a value R_(INT) preferably equal to the expected value R_(EXT) of the external resistor 462, although due to process and environmental variations, they are likely to be mismatched in actuality. Alternatively, the external resistor 462 need not be implemented off-chip. It may be an on-chip resistor which is laser trimmed to achieve an accurate and repeatable value, to which the other resistors (described below) are tuned.

Any current I_(TUNE) which flows through transistor 474 and internal resistor 464 produces a non-zero voltage on node 465, which is communicated to the noninverting input of operational amplifier 466. The operational amplifier 466 adjusts its output node 467 to a suitable voltage to adjust the current I_(TUNE) to produce on node 465 a voltage equal to the reference voltage V_(REF1). Consequently, I_(REF) R_(EXT) =I_(TUNE) R_(INT). It follows that:

I _(TUNE) /I _(REF) =R _(EXT) /R _(INT)  (Eq. 1)

In other words, the ratio of the two currents, I_(TUNE) to I_(REF), is the same as the resistor mismatch ratio, R_(EXT) to R_(INT).

The reference current I_(REF) which flows through transistor 472 is mirrored by transistor 473 to cause a current that is proportional to the reference current to flow through transistor 473, through a replica resistor 439 of the input termination resistor 412, to node 446 (which is biased by operational amplifier 434 as described herebelow). Similarly, the current I_(TUNE) which flows through transistor 474 is mirrored by transistor 475 to cause a current that is proportional to the current I_(TUNE) to flow through transistor 475, through a resistor 440 preferably having a resistance R₁ which is equal to the desired input termination resistor value (e.g., 100 ohms), and further to node 446. The two resistors 464 and 440 are preferably carefully implemented so that the ratio R₁/R_(INT) is accurately known.

The current proportional to I_(TUNE) flowing through resistor 440 establishes a reference voltage V_(REF2) which is communicated to the non-inverting input of amplifier circuit 442, which is preferably an operational amplifier circuit. This amplifier circuit 442 adjusts its output voltage signal V_(A) (conveyed on node 435) accordingly to adjust the resistance of transistor 424 in a direction to achieve a voltage on node 444 (being communicated to the inverting input of operational amplifier 442) which is equal to the reference voltage V_(REF2). As the output voltage signal V_(A) increases, the resistance of transistor 424 also increases (assuming P-channel transistors as depicted in this embodiment), and consequently the resistance of the replica termination resistor 439 also increases.

If the voltage V_(A) exceeds a predetermined high threshold level V_(REF)H, the output 451 of comparator 450 is driven high at the falling edge of a clock signal CLK, which asserts a decrement signal to the R_(DIG) control register 454. At the next rising edge of the clock signal CLK, the R_(DIG) control register 454 is decremented by one step. The various outputs of the R_(DIG) control register 454 are then inverted and driven onto the appropriate gate control signals conveyed on node 438.i by a corresponding plurality of “starved” inverters 436.i, thus increasing the resistance of the digital portion of the replica termination resistor 439. Preferably, a sufficient number of individual variable resistance circuits are included within the replica termination resistor 439 (and, of course, likewise within the input termination resistor 412) so that when decremented by one step, the value of the replica termination resistor 439 increases by approximately 6.25%, although other small increases may also be chosen. In the preferred embodiment, the R_(DIG) control register 454 produces a “thermometer coded” output. Seven such individual variable resistance circuits, having equal weights, will result in eight different resistance values and produce such a 6.25% increase or decrease per step. The analog control loop based on amplifier circuit 442 will then respond to this increase in resistance in the digital portion of the replica termination resistor 439 by driving the voltage of V_(A) lower, thereby decreasing the resistance of transistor 424 to offset the increase in the resistance of the digital portion. The speed of the amplifier circuit 442 loop is preferably fast enough, relative to the clock rate of CLK and to the slew rate of the digital gate control signals 438.i, so that the V_(A) voltage responds and is driven below the high threshold level V_(REF)H before the next falling edge of the clock signal CLK. Consequently, the decrement signal on node 451 is deasserted before the next rising edge of the clock signal CLK.

Similarly, if the voltage V_(A) falls below a predetermined low threshold level V_(REF)L, the output 453 of comparator 452 is driven high at the falling edge of the clock signal CLK, which asserts an increment signal to the R_(DIG) control register 454. At the next rising edge of the clock signal CLK, the R_(DIG) control register 454 is incremented by one step, thus decreasing the resistance of the digital portion of the replica termination resistor 439. The analog control loop based on amplifier circuit 442 will then respond to this decrease in resistance in the digital portion of the replica termination resistor 439 by driving the voltage of V_(A) higher, thereby increasing the resistance of transistor 424 to offset the decrease in the digital portion. As before, the speed of the amplifier circuit 442 loop is preferably fast enough, relative to the clock rate of CLK and to the slew rate of the digital gate control signals 438.i, so that the V_(A) voltage responds and is driven above the low threshold level V_(REF)L before the next falling edge of the clock signal CLK. Consequently, the increment signal on node 453 is deasserted before the next rising edge of the clock signal CLK.

It is possible that the V_(A) voltage may sufficiently exceed the high threshold level V_(REF)H such that, even when the R_(DIG) control register 454 is decremented once, the V_(A) voltage remains above the high threshold level V_(REF)H. In such a case the R_(DIG) control register 454 may be decremented during each of several consecutive clock cycles of the clock signal CLK, until the resistance of the digital portion of the replica termination resistor 439 is modulated enough to allow the V_(A) voltage to fall below the high threshold level V_(REF)H. Similarly, more than one consecutive increment cycle may result when the V_(A) voltage is well below the low threshold level V_(REF)L. Nonetheless, the circuit 400 eventually stabilizes at an operating point where the voltage of node 444 equals the reference voltage V_(REF2), and where V_(REF)L<V_(A)<V_(REF)H . At this point the following relationship holds:

∝I _(TUNE) R ₁ =∝I _(REF)(R _(REPLICA))

Consequently, it follows that:

I _(TUNE) /I _(REF)=(R _(REPLICA))/R ₁  (Eq. 2)

Equating (1) and (2) gives

R _(EXT) /R _(INT)=(R _(REPLICA))/R ₁

But since R₁ is a precise ratio of R_(INT), (i.e., R₁=βR_(INT)), then it follows that:

R _(REPLICA) =βR _(EXT)  (Eq. 3)

Since the input termination resistor 412 preferably matches the replica termination resistor 439, the input termination resistor 412 will have a value equal to a predetermined scaling factor times the value of the external resistor R_(EXT), and moreover will have the precision of the external resistor R_(EXT) (assuming that the R_(INT) resistor 464 and the R₁ resistor 440 are well enough matched that the scaling factor β is extremely accurately known).

It should be noted that the operational amplifier 434 drives its output node 446 to a voltage which results in node 438 of the replica termination resistor 439 having the same voltage as the common mode voltage V_(CM) (i.e., node 437) of the input termination resistor 412. This common mode voltage is generated by the voltage divider circuit formed, in this example, by resistors 430 and 431. Other resistance circuits may alternatively be employed to form such a voltage divider.

After the initial startup of the input impedance circuit 400 during which the initial values of the R_(DIG) control register 454 are set and the value of the V_(A) voltage is determined, the input termination resistor 412 is preferably maintained at its desired resistance value without any sudden changes or “glitches” in its value. This is a particularly desirable characteristic of this input impedance circuit 400 which follows if the drift of resistance value is slow relative to the clock rate of the clock signal CLK (which is almost assured), and if the analog portion of the input termination resistor 412 is fast enough to compensate for changes in the digital portion of the input termination resistor 412.

In tracking a parametric drift, when the V_(A) voltage is first driven beyond either the high threshold level V_(REF)H or the low threshold level V_(REF)L, then the R_(DIG) control register 454 will usually need to be decremented or incremented only once to bring the value of V_(A) back near the center of its dynamic range (and between the two threshold voltages). The R_(DIG) control register 454 output signals are buffered through “starved” inverters 436.i (i.e., 436.0, 436.1, . . . 436.N) to accomplish an extremely slow ramp-rate in whichever one(s) of the digital gate control signals 438.i happen to change states during an increment or decrement operation. This results in an extremely slow change in the resistance of the digital portion of the replica termination resistor 439, which affords the feedback loop based upon the amplifier circuit 442 (and to a lesser extent, operational amplifier 434) time to compensate for this change in resistance even during the (preferably slow) transition time of the digital gate control signals 438.i. In other words, the analog loop is fast enough that the total resistance value of the replica termination resistor 439 remains substantially constant even as its digital portion switches. As a result, the input termination resistor 412 value remains constant as the underlying resistor parameters drift, yet without any noticeable perturbations or transient effects which might otherwise disrupt the signal integrity of the incoming data input signal.

Other variations in such a circuit are contemplated, and several are described below. One such variation includes one or more voltage slow transition generators, such as a DAC, as described above in the context of the variable capacitance circuits, rather than using starved inverters. The weighting of the digital portion of both the input termination resistor 412 and the replica termination resistor 439, described above as a plurality of uniformly weighted variable resistance circuits may, of course, be implemented using other weighting values, including a plurality of binary-weighted variable resistance circuits responsive to a binary digital code generated by the R_(DIG) control register 454. Alternatively, a combination of non-identical and identically-weighted variable resistance circuits may be employed, which can be particularly advantageous in reducing required calibration times.

Another variation utilizes analog voltage control of each of the gate control signals (i.e., nodes 438.i) to vary the resistance of the corresponding variable resistance circuit. For example, the “power supply” voltage provided to each of the final buffers for the gate control signals (e.g., analogous to inverters 436.i) may instead be a controllable analog voltage which is varied to influence the voltage level of the “driven high” gate control signals. Whenever a new digital code is generated by the R_(DIG) control register 454, the analog voltage may be varied using an analog control loop to keep the total value of the replica termination resistor 439 constant (and likewise the input termination resistor 412 value constant).

In another embodiment, the external REX resistor may actually be implemented on-chip, and may be calibrated such as by laser trimming, digital control, or other methods to achieve a known resistance. One or more similar circuits to that described above may be employed to adjust each of a group of internal resistors to a precise ratio of the known resistance, rather than having to adjust each such internal resistor independently.

The above described input impedance circuit 400 illustrates one of the ways a resistor connected between two differential nodes may be sensed and adjusted. In other configurations having a more straightforward single-ended-type load resistor, particularly when connected between a power supply node (or another node having a relatively stable voltage) and an internal circuit node, various circuits useful to sense and adjust such a resistor may be easily implemented using the teachings set forth above. For example, a termination resistance circuit for a single-ended input may be accomplished by connecting node 406 and node 446 to a signal reference node, such as a signal ground node, and eliminating the common mode circuitry formed by resistors 430, 431, 432, and 433 and amplifier 434.

The feedback system just described may be generalized as shown in FIG. 21. A feedback system 500 includes a pair of tuning elements 504, 506 connected in parallel between nodes 508 and 510 which nodes are communicated to the remainder 502 of the feedback system. A first feedback loop is formed as a result of a first control signal conveyed on node 512 controlling the first tuning element 504. A digital control block 516, responsive to the first control signal conveyed on node 512, generates at least one output signal on node 514 that is coupled to control the second tuning element 576, thus forming a second feedback loop. An optional analog control block 518 may be included to generate the control signal for the tuning element 504 in response to the first control signal conveyed on node 512. In some embodiments, the same control signal conveyed on node 512 is coupled to both the digital control block 516 and to the analog control block 518 (if present), while in other embodiments the first (e.g., analog) feedback loop may respond to a separate control signal 522 and may utilize an optional analog control block 520. The feedback system 500 may include a second pair of tuning elements 534 and 536, connected in parallel between nodes 538 and 540, that are implemented to carefully match (or track with a well-controlled scaling factor) the tuning elements 504 and 506, respectively. The signal that controls the tuning element 504 is also coupled to the tracking tuning element 534 so that their behavior is well matched. Likewise, the signal(s) that controls the tuning element 506 (conveyed on node 514) is also coupled to the tracking tuning element 536 so that their behavior is also well matched. The aggregate impedance between nodes 538 and 540 is thus controlled by the feedback system 500, but the nodes 538 and 540 are themselves available to connect within another circuit. Additional circuits 530 (not shown in the figure) are also specifically contemplated, each including a pair of tuning elements analogous to tuning elements 534 and 536, and connected in parallel between a pair of circuit nodes, and which tuning elements are implemented to have a magnitude which is proportional to and carefully track the tuning elements 504 and 506, respectively. As used herein such an impedance may include a resistance, a reactance, or both.

Utilizing the teachings thus far described, a variety of different systems may be generalized which advantageously incorporate both a (preferably slow) digital feedback loop and an analog feedback loop. In another embodiment depicted in FIG. 22, a feedback system 550 includes an analog control block 554, responsive to a first control signal conveyed on node 558, whose output is coupled to a first control input 562 of the remainder 552 of the feedback system, thus forming a first feedback loop. The feedback system 550 further includes a digital control block 556, responsive to a second control signal conveyed on node 560, whose at least one output is coupled to a second control input 564 of the remainder 552 of the feedback system, thus forming a second feedback loop.

In another embodiment depicted in FIG. 23, a feedback system 570 includes a pair of tuning elements 574, 576 connected in parallel between nodes 578 and 580 which nodes are communicated to the remainder 572 of the feedback system. An analog control block 584 is responsive to a first control signal conveyed on node 582 and generates an output signal on node 586 for controlling the first tuning element 574, thus forming a first feedback loop. A digital control block 592, responsive to a second control signal conveyed on node 588, generates at least one output signal on node 590 that is coupled to control the second tuning element 576, thus forming a second feedback loop. In some embodiments, the first and second control signals 582 and 588 may be separate signals (as shown), while in other embodiment s they may be the same or substantially the same signal.

In yet another embodiment depicted in FIG. 24, a feedback system 600 includes a first tuning element 604 connected between nodes 614 and 616, which nodes are communicated to the remainder 602 of the feedback system. A second tuning element 606 is connected between nodes 624 and 626, which nodes are also communicated to the remainder 602 of the feedback system. An analog control block 610 is responsive to a first control signal conveyed on node 608 and generates an output signal on node 612 for controlling the first tuning element 604, thus forming a first feedback loop. A digital control block 620, responsive to a second control signal conveyed on node 618, generates at least one output signal on node 622 that is coupled to control the second tuning element 606, thus forming a second feedback loop. In some embodiments, the first and second control signals 608 and 618 may be separate signals (as shown), while in other embodiments they may be the same or substantially the same signal.

The block diagrams herein may be described using the terminology of a single node connecting the blocks. Nonetheless, it should be appreciated that, when required by the context, such a “node” may actually represent a pair of nodes for conveying a differential signal, or may represent multiple separate wires (e.g., a bus) for carrying several related signals or for carrying a plurality of signals forming a digital word.

In the interest of clarity, not all of the routine features of the implementations described herein are shown and described. It should, of course, be appreciated that in the development of any such actual implementation, numerous implementation-specific decisions must be made in order to achieve the developer's specific goals, such as compliance with application- and business-related constraints, and that these specific goals will vary from one implementation to another and from one developer to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking of engineering for those of ordinary skill in the art having the benefit of this disclosure.

Unless otherwise defined herein in the context of its usage, a significant perturbation of a parameter or condition, as used herein, is generally an amount which causes a circuit to materially behave differently. A perturbation of a parameter or condition which is large enough to result in the failure to meet a particular design specification is a significant perturbation of the parameter or condition.

Unless otherwise defined herein in the context of its usage, a first signal is related to a second signal if either is derived from the other, or if they are the same signal. Unless otherwise defined herein in the context of its usage, a capacitance circuit provides at least a capacitance between its terminals, but should not be narrowly interpreted to require a literal ‘capacitor’ element, although such a capacitor element is certainly contemplated. Other elements may be employed, such as transistor structures, to achieve a capacitance, and resistive components may also be present, even if only as a parasitic element. Similarly, unless otherwise defined herein in the context of its usage, a resistance circuit provides at least a resistive component of an impedance between its terminals, but should not be narrowly interpreted to require a literal ‘resistor’ element, although such an element is certainly contemplated. Other elements may be employed, such as transistor structures, particularly long-channel transistor structures, to achieve a resistance, and reactive components may also be present, even if only as a parasitic element. A capacitance circuit and a resistance circuit may include other structures, such as a switch circuit, as the context so requires. Unless otherwise defined herein in the context of its usage, a switch circuit may include any structure which can function as a switch, achieving a large change in resistance between its “on” state and “off” state in response to a control signal. For example, a single transistor, a full CMOS transfer gate structure, are but two examples of a wide variety of structures which may function as a switch circuit.

Unless otherwise defined herein in the context of its usage, a “nominal” value of an element, parameter, operating condition, or other aspect is generally the expected value of such element, which, if controllable by a control signal, may correspond to the mid-point or average value of the range of values, and if subject to variations resulting from semiconductor or environmental parameter variations, may correspond to the design target for a representative mid-value of the expected range. It should be understood that many such elements described as having a nominal value, may actually have, at any given time, a value that is different.

Based upon the teachings of this disclosure, it is expected that one of ordinary skill in the art will be readily able to practice the present invention. The descriptions of the various embodiments provided herein are believed to provide ample insight and details of the present invention to enable one of ordinary skill to practice the invention. Moreover, the various features and embodiments of the invention described above are specifically contemplated to be used alone as well as in various combinations.

Although certain implementation techniques (e.g., logic synthesis, etc.) are not specifically described, such techniques are well known, and no particular advantage is afforded by specific variations of such techniques in the context of practicing this invention. Conventional circuit design and layout tools may be used to implement the invention. The specific embodiments described herein, and in particular the various bit widths, register sizes, signed or unsigned format of certain digital values, and operating frequencies, are illustrative of exemplary embodiments, and should not be viewed as limiting the invention to such specific implementation choices.

The invention is contemplated to include circuits, systems of circuits, related methods, and computer-readable medium encodings of such circuits, systems, and methods, all as described herein, and as defined in the appended claims. As used herein, a computer readable medium includes at least disk, tape, or other magnetic, optical, semiconductor (e.g., compactflash cards, ROM), or electronic medium and a network, wireline, wireless or other communications medium. An encoding of a circuit may include circuit schematic information, physical layout information, behavioral simulation information, and/or may include any other encoding from which the circuit may be represented or communicated.

The foregoing details description has described only a few of the many possible implementations of the present invention. For this reason, this detailed description is intended by way of illustration, and not by way of limitations. Variations and modifications of the embodiments disclosed herein may be made based on the description set forth herein, without departing from the scope and spirit of the invention. It is only the following claims, including all equivalents, which are intended to define the scope of this invention. In particular, even though the preferred embodiments are described in the context of a PLL and an impedance tuning circuit, the teachings of the present invention are believed advantageous for use with other types of feedback systems. Moreover, the techniques described herein may also applied to other types of circuit applications. Accordingly, other embodiments, variations, and improvements not described herein are not necessarily excluded from the scope of the invention. 

What is claimed is:
 1. A controlled resistance circuit forming a resistance between a first node and a second node having a controlled magnitude proportional to a reference resistance, said circuit comprising: a first variable resistance circuit connected between the first node and the second node, said first variable resistance circuit having an analog portion responsive to an analog control signal and having a digital portion responsive to a digital control signal, and whose magnitude is responsive to both the analog control signal and the digital control signal; a second variable resistance circuit connected between a third node and a fourth node, said second variable resistance circuit having an analog portion responsive to the analog control signal and having a digital portion responsive to the digital control signal, and whose magnitude, for a given value of the analog control signal and a given value of the digital control signal, is proportional to the magnitude of the first variable resistance circuit; a first feedback circuit responsive to a voltage developed across the second variable resistance circuit by a controlled first current flowing therethrough, for generating the analog control signal accordingly to adjust the magnitude of the second variable resistance circuit toward a magnitude which is proportional to the reference resistance; a second feedback circuit responsive to the analog control signal when the analog control signal is driven outside a predetermined range, for generating the digital control signal to accordingly further adjust the magnitude of the second variable resistance circuit in like direction as that resultant from the analog control signal being driven outside the predetermined range, so that the first feedback circuit drives the analog control signal back within the predetermined range; said first and second feedback circuits being arranged to generally maintain the analog control signal within the predetermined range and to adjust the magnitude of the second variable resistance circuit to a magnitude proportional to the reference resistance, thereby likewise adjusting the magnitude of the first variable resistance circuit to a controlled magnitude which is proportional to the reference resistance.
 2. The invention as recited in claim 1 wherein the second feedback circuit is configured to affect the magnitude of the first and second variable resistance circuits substantially more slowly than does the first feedback circuit, so that the first and second feedback circuits together control the magnitude of the first and second variable resistance circuits without significant perturbation.
 3. The invention as recited in claim 1 wherein the first, second, third, and fourth nodes are all different nodes.
 4. The invention as recited in claim 1 wherein the second and fourth nodes are coupled to a signal reference node.
 5. The invention as recited in claim 1 wherein the reference resistance comprises a resistor external to an integrated circuit upon which the controlled resistance circuit is implemented.
 6. The invention as recited in claim 1 wherein the reference resistance comprises a resistor integral to an integrated circuit upon which the controlled resistance circuit is implemented.
 7. The invention as recited in claim 6 wherein the integral reference resistance is trimmable during manufacture or testing of the integrated circuit upon which the controlled resistance circuit is implemented.
 8. The invention as recited in claim 1 further comprising: a third variable resistance circuit connected between a fifth node and a sixth node, said third variable resistance circuit having an analog portion responsive to the analog control signal and having a digital portion responsive to the digital control signal, and whose magnitude, for a given value of the analog control signal and a given value of the digital control signal, is proportional to the magnitude of the second variable resistance circuit, for providing a controlled resistance between the fifth and sixth nodes which is proportional to the reference resistance.
 9. The invention as recited in claim 8 wherein the first, second, third, fourth, fifth, and sixth nodes are all different nodes.
 10. The invention as recited in claim 8 wherein the second, fourth, and sixth nodes are coupled to a signal reference node.
 11. The invention as recited in claim 1 wherein: the digital control signal comprises a plurality M of digital switch control signals forming a multiple-bit digital word; the digital portion of the first variable resistance circuit comprises a plurality M of individual resistance circuits, each having a respective weight and each responsive to a corresponding one of the plurality M of digital switch control signals, and each having a first terminal coupled to the first node and a second terminal coupled to the second node; and the digital portion of the second variable resistance circuit comprises a plurality M of individual resistance circuits, each having a respective weight and each responsive to a corresponding one of the plurality M of digital switch control signals, and each having a first terminal coupled to the third node and a second terminal coupled to the fourth node.
 12. The invention as recited in claim 11 wherein the respective weight of each of the plurality M of individual resistance circuits are substantially equal.
 13. The invention as recited in claim 11 wherein the respective weight of each of the plurality M of individual resistance circuits are not all substantially equal.
 14. The invention as recited in claim 11 wherein the respective weight of each of the plurality M of individual resistance circuits are binary weighted.
 15. The invention as recited in claim 11 wherein: the second feedback circuit is configured to cause no more than one of the plurality of digital switch control signals to transition at a given time.
 16. The invention as recited in claim 11 wherein: the second feedback circuit is configured to cause, at times, more than one of the plurality of digital switch control signals to simultaneously transition.
 17. The invention as recited in claim 11 wherein: the second feedback circuit is configured so that each of the plurality of digital switch control signals transitions between a first quiescent level and a second quiescent level with a controlled slow transition time.
 18. The invention as recited in claim 11 wherein: the second feedback circuit is configured to cause no more than one of the plurality of digital switch control signals to transition at a given time, and to cause each of the plurality of digital switch control signals to transition between a first quiescent level and a second quiescent level with a controlled slow transition time.
 19. The invention as recited in claim 11 wherein: each individual resistance circuit includes a switch circuit having a switch control terminal coupled to receive a corresponding one of the plurality of digital switch control signals.
 20. The invention as recited in claim 19 wherein: each individual resistance circuit further comprises at least one resistor circuit coupled between one of the respective terminals of the individual resistance circuit and the respective switch circuit.
 21. The invention as recited in claim 19 wherein each individual resistance circuit further comprises: a first resistor circuit coupled between one of the respective terminals of the individual resistance circuit and the respective switch circuit; and a second resistor circuit coupled between the other of the respective terminals of the individual resistance circuit and the respective switch circuit.
 22. The invention as recited in claim 3 wherein: the first and second nodes comprise a differential input for an integrated circuit; and the first variable resistance circuit comprises an internal termination resistance for the differential input.
 23. The invention as recited in claim 4 wherein: the first node comprise a single-ended input for an integrated circuit; and the first variable resistance circuit comprises an internal termination resistance for the single-ended input.
 24. The invention as recited in claim 1: wherein the first variable resistance circuit includes a first voltage divider circuit connected between the first and second nodes to generate a first common mode voltage on an intermediate node therebetween; wherein the second variable resistance circuit includes a second voltage divider circuit connected between the third and fourth nodes to generate a second common mode voltage on an intermediate node therebetween; and further comprising an amplifier circuit responsive to the first and second common mode voltages for driving the fourth node to a voltage which results in the second common mode voltage being substantially equal in magnitude to the first common mode voltage.
 25. The invention as recited in claim 11 wherein second feedback circuit comprises: comparison circuitry for generating at least one out-of-range (OOR) signal when the analog control signal is driven outside its predetermined range; a digital control circuit responsive to the at least one OOR signal for increasing or decreasing the number of digital switch control signals which are driven to an active level.
 26. The invention as recited in claim 25 wherein the comparison circuitry is configured to generate a first out-of-range (OOR) signal when the analog control signal is driven in a first direction outside its predetermined range and for generating a second out-of-range signal when the analog control signal is driven in a second direction opposite the first direction outside its predetermined range.
 27. The invention as recited in claim 25 wherein the digital control circuit is arranged to transition each of the plurality of digital switch control signals between a first quiescent level and a second quiescent level with a controlled slow transition time.
 28. The invention as recited in claim 27 wherein the digital control circuit comprises: starved driver stages configured to drive the plurality of digital switch control signals between a first quiescent level and a second quiescent level with a deliberately degraded transition time relative to a normally sized driver stage.
 29. The invention as recited in claim 27 wherein the digital control circuit comprises: at least one slow transition generator having an output signal sharable among more than one of the plurality of digital switch control signals to drive at least a corresponding one of the plurality of digital switch control signals between a first quiescent level and a second quiescent level with a controlled slow transition time.
 30. The invention as recited in claim 29 wherein the at least one slow transition generator comprises: a digital to analog converter (DAC) responsive to a multi-bit digital control word that is sequenced through a plurality of digital values to generate a slowly decreasing or increasing value on an output thereof.
 31. The invention as recited in claim 1 wherein the first feedback circuit comprises: a second resistor circuit having a particular magnitude and coupled between the fourth node and a seventh node; a current source circuit for generating a voltage across the second resistor circuit; an amplifier circuit responsive to a voltage differential between the fourth and seventh nodes, for generating the analog control signal in accordance therewith.
 32. The invention as recited in claim 11 wherein the analog portion of the second variable resistance circuit comprises: at least one individual variable resistance circuit, each responsive to the analog control signal, each having a first terminal coupled to the third node and a second terminal coupled to the fourth node.
 33. A computer readable medium encoding a circuit for providing a resistance between a first node and a second node having a controlled magnitude proportional to a reference resistance, said encoded circuit comprising: a first variable resistance circuit connected between the first node and the second node, said first variable resistance circuit having an analog portion responsive to an analog control signal and having a digital portion responsive to a digital control signal, and whose magnitude is responsive to both the analog control signal and the digital control signal; a second variable resistance circuit connected between a third node and a fourth node, said second variable resistance circuit having an analog portion responsive to the analog control signal and having a digital portion responsive to the digital control signal, and whose magnitude, for a given value of the analog control signal and a given value of the digital control signal, is proportional to the magnitude of the first variable resistance circuit; a first feedback circuit responsive to a voltage developed across the second variable resistance circuit by a controlled first current flowing therethrough, for generating the analog control signal accordingly to adjust the magnitude of the second variable resistance circuit toward a magnitude which is proportional to the reference resistance; a second feedback circuit responsive to the analog control signal when the analog control signal is driven outside a predetermined range, for generating the digital control signal to accordingly further adjust the magnitude of the second variable resistance circuit in like direction as that resultant from the analog control signal being driven outside the predetermined range, so that the first feedback circuit drives the analog control signal back within the predetermined range; said first and second feedback circuits being arranged to generally maintain the analog control signal within the predetermined range and to adjust the magnitude of the second variable resistance circuit to a magnitude proportional to the reference resistance, thereby likewise adjusting the magnitude of the first variable resistance circuit to a controlled magnitude which is proportional to the reference resistance.
 34. The invention as recited in claim 33 wherein: the digital control signal comprises a plurality M of digital switch control signals forming a multiple-bit digital word; the digital portion of the first variable resistance circuit comprises a plurality M of individual resistance circuits, each having a respective weight and each responsive to a corresponding one of the plurality M of digital switch control signals, and each having a first terminal coupled to the first node and a second terminal coupled to the second node; and the digital portion of the second variable resistance circuit comprises a plurality M of individual resistance circuits, each having a respective weight and each responsive to a corresponding one of the plurality M of digital switch control signals, and each having a first terminal coupled to the third node and a second terminal coupled to the fourth node.
 35. The invention as recited in claim 34 wherein the second feedback circuit is configured to cause no more than one of the plurality of digital switch control signals to transition at a given time, and to cause each of the plurality of digital switch control signals to transition between a first quiescent level and a second quiescent level with a deliberately slowed transition time.
 36. The invention as recited in claim 33 wherein: the first and second nodes comprise a differential input; and the first variable resistance circuit comprises a termination resistance for the differential input.
 37. A resistance circuit providing a resistance between a first node and a second node having a controlled magnitude proportional to a reference resistance, said circuit comprising: a first variable resistance circuit connected between the first node and the second node, said first variable resistance circuit having a magnitude that is responsive to at least one control signal; a second variable resistance circuit connected between a third node and a fourth node, said second variable resistance circuit having a magnitude that is responsive to the at least one control signal and which magnitude, for a given value of the at least one control signal, is proportional to the magnitude of the first variable resistance circuit; a bias circuit including a first bias resistance, said bias circuit for generating a controlled reference current and a controlled bias current, wherein a ratio of the controlled bias current to the controlled reference current is proportional to the ratio of the reference resistance to the first bias resistance; a second bias resistance having a magnitude which is proportional to that of the first bias resistance and which is implemented to track variations in the first bias resistance; a feedback circuit arranged to compare a first voltage developed across the second bias resistance by the controlled bias current flowing therethrough against a second voltage developed across the second variable resistance circuit by the controlled reference current flowing therethrough, and to generate accordingly the at least one control signal to adjust the value of the second variable resistance circuit so that the first voltage substantially equals the second voltage, thereby resulting in the second variable resistance circuit having a magnitude proportional to the reference resistance; whereby the first variable resistance circuit, being also responsive to the at least one control signal, likewise achieves a magnitude proportional to the reference resistance.
 38. The invention as recited in claim 37 wherein the first, second, third, and fourth nodes are all different nodes.
 39. The invention as recited in claim 37 wherein the second and fourth nodes are coupled to a signal reference node.
 40. The invention as recited in claim 37 wherein: the reference resistance comprises a resistor external to an integrated circuit upon which the controlled resistance circuit is implemented; the first bias resistance is implemented having a nominal value proportional to the nominal value of the external reference resistor; and the second bias resistance is implemented having a nominal value proportional to a desired value of the second variable resistance circuit.
 41. The invention as recited in claim 37 further comprising: a third variable resistance circuit connected between a fifth node and a sixth node different from the first, second, third, and fourth nodes, said third variable resistance circuit having a magnitude that is responsive to the at least one control signal and which magnitude, for a given value of the at least one control signal, is proportional to the magnitude of the second variable resistance circuit, for providing a controlled resistance between the fifth and sixth nodes which is proportional to the reference resistance.
 42. The invention as recited in claim 37 wherein the second variable resistance circuit comprises: at least one individual variable resistance circuit responsive to an analog control signal, having a first terminal coupled to the third node and a second terminal coupled to the fourth node; and a plurality of individual variable resistance circuits, each responsive to a corresponding one of a plurality of digital control signals, and each having a first terminal coupled to the third node and a second terminal coupled to the fourth node.
 43. The invention as recited in claim 42 wherein the feedback circuit comprises: a control circuit for generating each of the plurality of digital control signals and configured to cause no more than one of the plurality of digital control signals to transition at a given time, and further configured to cause each of the plurality of digital control signals to transition between a first quiescent level and a second quiescent level with a deliberately slowed transition time.
 44. The invention as recited in claim 42 wherein each individual variable resistance circuit comprises: a switch circuit having a switch control terminal coupled to receive a corresponding one of the analog or digital control signals; a first resistor circuit coupled between one of the respective terminals of the individual variable resistance circuit and the respective switch circuit; and a second resistor circuit coupled between the other of the respective terminals of the individual variable resistance circuit and the respective switch circuit.
 45. The invention as recited in claim 37 wherein: the first variable resistance circuit comprises an internal termination resistance for the differential input signal.
 46. The invention as recited in claim 37: wherein the first variable resistance circuit includes a first voltage divider circuit connected between the first and second nodes to generate a first common mode voltage on an intermediate node therebetween; wherein the second variable resistance circuit includes a second voltage divider circuit connected between the third and fourth nodes to generate a second common mode voltage on an intermediate node therebetween; and further comprising an amplifier circuit responsive to the first and second common mode voltages for driving the fourth node to a voltage which results in the second common mode voltage being substantially equal in magnitude to the first common mode voltage.
 47. The invention as recited in claim 42 wherein the feedback circuit comprises: comparison circuitry for generating a first out-of-range (OOR) signal when the analog control signal is driven in a first direction outside its predetermined range and for generating a second out-of-range signal when the analog control signal is driven in a second direction opposite the first direction outside its predetermined range; a digital control circuit responsive to the first and second OOR signals for increasing or decreasing the number of digital control signals which are driven to an active level.
 48. The invention as recited in claim 43 wherein the control circuit comprises: starved driver stages configured to drive the plurality of digital control signals between a first quiescent level and a second quiescent level with a deliberately slowed transition time.
 49. The invention as recited in claim 43 wherein the control circuit comprises: at least one slow transition generator having an output signal shared among more than one of the plurality of digital control signals to drive a corresponding one of the plurality of digital control signals between a first quiescent level and a second quiescent level with a deliberately slowed transition time.
 50. The invention as recited in claim 49 wherein the at least one slow transition generator comprises: a digital to analog converter (DAC) responsive to a multi-bit digital control word that is sequenced through a plurality of digital values to generate a slowly decreasing or increasing value on an output thereof.
 51. A computer readable medium encoding a circuit, said encoded circuit as defined in claim
 37. 